A High-Speed Feedthrough (HSFT) is a circuit structure that provides electrical connectivity over RF transmission lines having two displaced sets of terminals. HSFTs may form part of an electronic multi-chip module (MCM) package where the HSFT provides electrical connectivity between the hermetically sealed interior of the package and the external system or host device. When a HSFT is used in a MCM package, one set of terminals is connected to the MCM internally within the package while the other set of terminals are connected to the host device externally from the package. The HSFT may also form part of the hermetic separation of the package from the host device.
For high-speed applications operating at frequencies of 10 GHz and above, such as opto-electronic telecommunication components, a HSFT should provide compact broadband resonance-free transmission with low losses, minimal inter-channel cross-talk, and small reflection from terminals on both sides of the package.
When designing a HSFT, significant challenges arise when the HSFT must provide connectivity in two dimensions—vertically and horizontally—between its two sets of terminals. Coplanar HSFTs limit the HSFT structure such that all of the terminals and RF transmission lines must lie in the same plane, e.g., the horizontal plane. Accordingly, connectivity in both the horizontal and vertical dimensions is not possible using a coplanar HSFT. While some non-coplanar HSFTs exist for specific scenarios, there has been no universal solution that can accommodate vertical and horizontal displacements that are comparable with, or longer than, half of the shortest wavelength transmitted through the HSFT.
FIG. 1A illustrates an example HSFT 100 having coplanar transmission lines within the same layer. The HSFT 100 in FIG. 1A is described as a coplanar HSFT because the transmission lines are arranged either as coplanar lines or as coplanar buried strip lines on or within the same layer. Coplanar HSFTs are not well suited when there is also vertical displacement between the two sets of terminals.
A non-coplanar HSFT refers to a HSFT where there is vertical displacement between the input and output sets of terminals in addition to horizontal displacement, if any. Some non-coplanar HSFTs, such as described in US Patent Application Publication No. 2009/0033442 and US Patent Application Publication No. 2009/0267712, include a single vertical interconnect, or via, spanning the vertical displacement in each RE transmission line.
Non-coplanar HSFTs having a single via are not well suited when the vertical and horizontal displacements between the two sets of terminals are comparable with, or longer than, half of the shortest wavelength transmitted through the HSFT. A distance, length or height is comparable with, proportional to, or approximately half of a wavelength when the distance, length or height is within the range of about 25% to about 75% of the wavelength.
In FIG. 1B a single via non-coplanar HSFT 150 is illustrated within a package 160. The package 160 includes a hermetic seal 170. For simplicity, only one RF transmission line is illustrated in FIG. 1B although others could be present. A single via spans multiple layers of the non-coplanar HSFT 150. This via cannot be described as an ideal microwave transmission line. Furthermore, when the vertical length of the via is comparable with, or longer than, half of the effective wavelength in the substrate material (λef), the via fails due to resonances and excessive cross-talk. Accordingly, non-coplanar HSFTs having a single via, including HSFT 150, are not well suited when the vertical and horizontal displacements between the two sets of terminals are comparable with, or longer than, half of the shortest wavelength transmitted through the HSFT.
Another non-coplanar HSFT design uses a sequence of vias without horizontal traces. The via in each ceramic or substrate layer is shifted horizontally while still partially overlapping with the vias in the layers above and below it. This design avoids resonance from a long single via; however, it cannot easily accommodate horizontal displacement. Since a via diameter is typically quite small, many ceramic layers (typically much more than 10, and preferably as much as 40) are required to span the horizontal displacement between the HSFT's sets of terminals. This design is not compact due to the large number of layers required. Furthermore, this design is expensive because it requires precise positioning of via channels and metallization in each layer. Accordingly, non-coplanar HSFTs having a sequence of progressively shifted vias are not well suited when the vertical and horizontal displacement between the two sets of terminals are comparable with, or longer than, half of the shortest wavelength transmitted through the HSFT.
A variant of the coplanar/non-coplanar HSFTs is described in U.S. Pat. No. 6,369,324. In that patent, each RF transmission line includes a pair of vias in the same substrate layer which route the RF transmission line beneath a hermetic sealing wall to improve the reliability of the hermetic seal. This patent does not appear to contemplate designing an HSFT having vertical and horizontal displacements between its two sets of terminals that are comparable with, or longer than, half of the shortest wavelength transmitted through the HSFT.